This invention relates to a system for the fractionation of a gaseous mixture, especially hydrocarbon mixtures, by distillation, especially at low temperatures wherein either components of the gaseous mixture form a substantially azeotropic mixture or at least one of the components of the gaseous mixture tends to freeze out.
By way of definition, the term "distillation" used herein is intended to embrace rectification as well as other conventional distillation processes. The term "substantially azeotropic mixture" is intended to embrace true azeotropic as well as approximate azeotropic mixtures, e.g., equilibrium mixtures of CO.sub.2 with H.sub.2 S which at certain constant pressures exhibit CO.sub.2 and H.sub.2 S concentrations in the vapor phase that are everywhere greater than those in the liquid phase, but where the concentrations of CO.sub.2 and H.sub.2 S in the vapor and liquid phases are nearly identical over an extended composition range.
Fractionation processes pertinent to this invention have been described in "Hydrocarbon Processing", May 1982, pp. 131-136. If a gaseous mixture is to be fractionated wherein the components during distillation form a substantially azeotropic mixture, then product purity is limited by the concentration of the components at the azeotropic point. In the distillation of other gaseous mixtures at low temperatures, where the reflux liquid is insufficient to maintain all of the components in solution at the cold temperatures, one or more of these components begin to freeze out, thereby terminating the distillation.
One suggested solution to the problem proposed in the aforementioned prior publication resides in admixing to the reflux liquid in the column an additional component, such as n-butane or a mixture of light or heavy hydrocarbons. Such admixture has the effects of increasing the amount of reflux liquid, altering the composition of the liquid, and increasing the temperature in the column. These effects, in the one case, suppress the deposition of solids which would otherwise freeze out, and in the other case with azeotropic mixtures results in an improvement of the purity of the fractionation products. The added components are then withdrawn from the distillation column and recovered in a supplemental distillation stage.
This suggested process, however, has several disadvantages. For example, in the separation of the additional component in the downstream supplemental distillation stage another azeotropic mixture occurs in part, for example ethane and carbon dioxide. Consequently, an additional component must also be introduced into this supplemental downstream distillation stage and this increases the cost of the process.
A further disadvantage resides in that the additionally introduced component causes dilution of the reflux liquid in the column, thus increasing the work of separation in the column. Still another disadvantage is that the additional component that must be fed to the column as a cold liquid is heated, during its downward flow within the column, to the higher temperature of the liquid in the column sump, so that a large portion of the cold value is dissipated.